Internal Insulation


Why insulate?

  • increased comfort
  • reduced maintenance
  • reduce costs for inhabitants
  • beneficial for the environment.

Where to use it?
In general, cavity fill is the most cost-effective way to improve the insulation of brick or masonry cavity walls (for details, see ‘Cavity Wall insulation in existing housing’ GPG 26). On solid walls, insulation can be improved by installing either internal dry lining or external cladding. In most cases, a dry lining is preferred because it costs less to install and maintain, with no scaffolding or special skills required. It also has advantages that materials are readily available and the building’s external appearance is maintained. Internal wall insulation is especially suitable when it is installed:

  • As part of a refurbishment that involves the disruption of internal surfaces and fixtures.
  • In multi-storey buildings where access for external insulation would be expensive.
  • In buildings with an attractive external appearance and/or in conservation areas
  • In high thermal capacity structures that have problems of condensation and mould growth.

Where not to use it?
Internal insulation may be inappropriate where:

  • Ornate plasterwork or wood panelling needs to be retained, and/or the interior appearance is protected under a listed buildings order
  • It is difficult to avoid thermal bridging
  • Walls are unsuitable because of rain penetration or the risk of summer condensation
  • Room sizes are too small to accommodate the reduction caused by the thicker internal lining
  • The disruption of fixtures or services would be excessive
  • The cost of disrupting or temporarily housing the occupants is too great.

In the above circumstances, alternative forms of insulation will need to be considered and insulation may need to be increased in other areas.

Regulations
Building regulations for the thermal insulation of walls vary between the nations. Advice should be sought from building control in the local authority. Copies of regulations are available from the Stationery Office.

Methods

Internal Insulation Methods
The main internal insulation methods are described below. However, the manufacturer’s instructions should be referred to for precise details of fixing to backgrounds of different strengths, wetness and material.

Method 1
Plasterboard thermal laminate, incorporating a vapour control layer between the plasterboard and insulation, fixed in a variety of ways:

  • secured to the wall with adhesive
  • screwed to metal furrings bonded to wall with plaster dabs
  • nailed or screwed to timber battens fixed to the wall
  • nailed or screwed to timber framed walling
  • screwed to smooth dry walls.

1200 Perimeter of board sealed with continuous fillet of plaster adhesive
Screw fixings length to suit floor to ceiling height. The number and spacing of fixings varies. Check with manufacturer’s specification. All dimensions are in millimetres
Bands of plaster adhesive existing plaster in good condition new plasterboard thermal laminate with integral vapour control layer bonded with plaster adhesive to wall laminates may also be fixed with plaster dabs, or screwed directly to brickwork or to metal furrings
Existing solid wall
Screw fixings 35mm min into brickwork
Image 1: Plasterboard thermal laminate
Iamge 2: Timber framed external wall with plasterboard laminate
Existing timber sheathing
Existing timber weather boarding with ventilation behind
Existing timber frame with plasterboard removed to allow
inspection and preservative treatment of timber structure
New plasterboard thermal laminate with integral vapour control layer,
nailed to timber studs if suitable, extra insulation should be added to cavity.
Note: An extra layer of plasterboard may be required for fireproofing. This should be fitted directly to the studs.
Internal wall insulation in existing housing

Method 2
Rigid insulation, friction-fitted between battens, fixed to the wall then covered with a vapour control layer or with flanged paper faced quilt insulation, incorporating a vapour control layer stapled to battens, then finished with plasterboard.

Method 3
Timber frame, braced between the floor and ceiling and kept clear of the external wall. Insulation stapled to the frame with a vapour control layer and plasterboard finish.

Method 4
Aluminium-faced polyethylene air bubble sheet on horizontal battens, fixed to the external wall, with vertical counter-battens, fixed over the insulation to receive the plasterboard finish

Figure 3: Timber stud frame inside solid no fines wall
Plasterboard
Existing no-fines wall
Vertical timber battens with horizontal members, plugged to wall rigid insulation friction fitted between battens polyethylene vapour control layer alternative insulation can be flanged paper faced quilt with integral vapour control layer

Figure 5: Polyethylene air bubble sheet
Existing solid wall
Existing plaster
Plasterboard
Air bubble sheet stapled to horizontal battens vertical battens spaced to suit
plasterboard sheets, fixed over air bubble sheets horizontal battens at
about 600mm centres

Figure 4: Insulation fitted to battens existing solid wall with external render
Timber studs braced floor to ceiling – spaced to suit board dimensions. Flanged paper-faced insulation quilt with integral vapour control layer stapled to frame studs clear of wall by min 30mm plasterboard
Internal wall insulation in existing housing

Choice of Method
There is a wide range of insulating materials available for dry lining using the four basic methods covered in this guide. Indicative properties of insulating materials are set out in and gives U-values of typical construction.

Adhesive fixing (sub)
Where the wall is dry and has a sound finish of plaster or concrete, laminate boards can be fixed directly to the wall with an adhesive recommended by the board manufacturer.  The adhesive is normally applied in 200mm wide bands approximately 50mm from the vertical edges and in the centre of the board. Bands of adhesive are also applied around the periphery of the wall and window to ensure continuous contact at the edges of the boards. To prevent collapse in the event of fire, boards that incorporate insulation made from plastics should be secured with screw fixings and fixed at least 35mm into the solid background. The number of fixings required will be specified by the board manufacturer; some will require nine fixings, and some only two. Positioned as shown in figure 1 on dry, even walls, mechanical fixing may be used without adhesive, subject to manufacturers’ recommendations.

Plaster dab fixing
If the wall is uneven, for instance where it has been stripped of plaster, bonding thermal laminate to the wall with plaster based adhesive dabs or strips is a suitable method providing the wall is dry. The perimeter of the wall and the surround of any openings, such as windows, must be sealed with a continuous band of plaster adhesive to restrict air movement into the building. This would otherwise reduce the thermal effectiveness of the insulation. It is important that sufficient adhesive is used for the perimeter seals to make continuous contact with both the wall and insulation. Mechanical fixings, as for adhesive fixed boards, are also needed.

Fixing to metal furrings or timber battens
Walls that are uneven can also be insulated, by first fixing metal furrings or preservative-treated timber battens to the walls and then screwing or nailing, thermal laminate onto them. All four edges and the centre of the laminate require support. This method is suitable for walls that are temporarily damp (following treatments for rising damp, after curing rain penetration, or after taking measures to reduce condensation). Non-traditional buildings, constructed with internal timber or metal frames, can also be insulated by nailing thermal laminate directly onto existing timber (provided it is in good condition) or onto a timber frame supported by the metal frame. Existing vapour control layers, if retained incorrectly within the construction, may cause condensation within the structure and should be removed. For this reason, the components of the existing wall (including any residual insulation) should be determined and a condensation calculation carried out, taking into account all the layers of the final construction.

Insulation between battens
Walls can be insulated between vertical timber battens, commonly 50mm wide and at least the thickness of the insulation, fixed to the wall at centres to suit the width of plasterboard. The boards should be supported by battens at the floor and ceiling, and by intermediate noggins at the middle of the board. All timber should be preservative-treated. Rigid insulation boards can be fitted between the battens or flanged paper-faced quilt insulation can be stapled to the battens. A continuous vapour control layer should be fixed on the warm side of the insulation. This layer may be a separate polyethylene sheet stapled to the battens or incorporated within the flanged paper facing to rolls of insulation (figure 3).

Constructing a separate inner lining
If it is difficult to achieve a secure fixing to the wall, or if the wall is damp, an independent timber or metal inner frame can be constructed leaving a cavity behind the external wall. The frame, supported at floor and ceiling level, is usually 50mm x the thickness of the insulation preservative-treated timber, or metal channel and I-sections dimensioned to take plasterboard laminates. Insulation is fixed (as in ‘Insulation between battens’) between the timber studs, and covered with a vapour control layer and plasterboard or, for additional thermal performance, a thermal laminate incorporating a vapour control layer (figure 4).

Air bubble sheets
This system is suitable for most walls. The air bubble sheet is fixed to horizontal preservative-treated battens plugged to the external wall. The bubble sheet is then fixed to this framework taking care to lap and welt or tape joints between sheets of the material.  Vertical battens are fixed over the insulation at appropriate centres to support the plasterboard. Plasterboard fixed in contact with the insulation without the secondary battens is less thermally effective and is not recommended.